It didn't hit him but did fly behind him.
The Secret Service says it was aware of the incident when it happened.
A spokesman says the book was thrown by an "over-exuberant" individual who wanted Obama to have a copy of a book he had written.
The Secret Service says it interviewed the book thrower and deemed there was no threat - just bad judgment on his part. No arrest was made and the Service declined to give his name.
At the same rally, a streaker was arrested. Apparently, the nude man was trying to win $1 million in a publicity stunt for a website.
The streaker wasn't involved in the book throwing, says the Secret Service.
CBS News correspondent Jeff Glor told the story on "The Early Show":